Magnetometers are generally used for determining the existence of, and the magnitude of, static or fluctuating or alternating magnetic fields. Some useful applications include spacecraft attitude determination, navigation using Earth's magnetic field, and remote detection of magnetic objects, such as submerged submarines.
An ordinary magnetic compass with a “floating” magnetic needle is a form of magnetometer, which has likely been used for at least a thousand years to indicate the local direction of the Earth's magnetic field. The magnetized needle has a low-energy state when it is aligned with the local magnetic field.
Other types of magnetometers include various inductor-based arrangements including flux-gate magnetometers, in which a varying excitation voltage is applied to a first coil to produce a magnetic field of varying amplitude and a separate differential secondary coil senses the magnetic field produced by the first coil to thereby generate an output voltage which depends upon the excitation voltage and any intervening magnetic field.
A prior-art electron-spin-detection magnetometer is described in U.S. Pat. No. 5,189,368, issued Feb. 23, 1993 in the name of W. E. Chase. In the Chase magnetometer, describes an electron spin magnetometer in which a first pulsed light source of suitable wavelength excites electron spin precession about the incident magnetic field vector and also results in a phase change between electron quantum energy spin (+½ and −½) depending upon the magnitude of the magnetic field. Upon excitation with a second pulse of light, the electron orbits are driven to their ground state, which results either in the release of photons or in grouping of atoms in populations having the +½ and −½ spin states, which thereafter decay with time depending upon the nature of the material. The Chase magnetometer uses a third signal or first interrogating signal, which may be a continuous radio-frequency (RF) carrier, which is modulated by the decaying electron spins after the second excitation light pulse, from which modulation the magnitude of the magnetic field may be determined.
An article by J. M. Taylor et al, which appeared at pp 810-816 in the magazine Nature physics, Vol. 4, Oct. 2008, entitled High-sensitivity diamond magnetometer with nanoscale resolution describes some prior art and physics of magnetometry. The Taylor et al. article also describes detection of weak magnetic fields taking advantage of coherent control of solid-state electron spin quantum bits. The Taylor et al. magnetometer makes use of solid crystal detection material, which is advantageous in that the effective sensitivity is greater than that of gas-based systems because of the greater density of the sites available for electron excitation.
The principles of magnetometry in the prior art may be explained in conjunction with FIG. 1A, in which a solid-state crystal sensor is illustrated as a block 10. Sensor 10 is composed of a chiral atomic structure allowing electron spin response and subsequent spin alignment with an incident magnetic field, illustrated by an arrow B(t). As described by Taylor et al., the crystal may be carbon atoms in a crystal (diamond) structure, with nitrogen impurities. FIG. 1B illustrates a diamond crystal with carbon atoms, some of which are designated 3. The crystal lattice of FIG. 1B also illustrates a vacancy (V) designated 4, resulting from an offset nitrogen atom 5. In FIG. 1C, the direction of the external or incident magnetic field is illustrated by B(t) arrow 6, and this direction is normal to the “vacancy axis”. The vacancy axis is displacement vector extending from the vacancy point to the displaced nitrogen atom. Also in FIG. 1C, curved arrow 7 shows the direction of precession of the nitrogen atom in response to magnetic field B(t). In the absence of excitation of the “vacancy electrons” (electrons of the carbon atoms surrounding the vacancy), their quantum energy levels are identical, as suggested by the equal lengths of energy-level-representative arrows 8.
FIG. 1D is similar to FIG. 1C, and shows the result of a first excitation pulse 9 of light at a wavelength of 532 nanometers (nm) and with a duration of T seconds. The wavelength of the light pulse is selected to excite electron spin precession about the incident magnetic field B(t), which also results in a corresponding phase change between electron quantum energy spin (+½ or −½) spin states. The phase differences in the spin states of the vacancy electrons resulting from this excitation are illustrated as a two-headed arrow 10. The phase difference of the vacancy's quantum electron energy levels is proportional to the spin precession. FIG. 1E illustrates application of a second pulse 11 of green light at the same 532 nm wavelength and with duration T seconds. The excitation of the electron states attributable to the second pulse is out-of-phase with the excitation attributable to the first pulse. That is, the timing T of the second pulse relative to the first pulse is selected so that the previously excited electron states 10 are driven to ground energy level or state. As a result of the transition to the ground state, the electrons emit red-light photons illustrated as 12, and the number of the red-light photons is proportional to the phase difference, which in turn is proportional to the magnitude of the incident magnetic field B(t).
FIG. 1F illustrates the photons of red light 12 applied to a charge-coupled two-dimensional imaging array device illustrated as 13 within an apparatus 210 according to an aspect of the invention. Imaging device 13 includes n cells, and converts the incident red light into electrical signal in the form of charge. In theory, it would be possible to use a single photodetector element, but as the light radiation leaving the crystal sensor may not be confined to a single direction, the detection must take place over a finite area. The photon-representative electron charge is coupled over n parallel paths 212 to an accumulator or integrator illustrated as a block 14, which accumulates the charge for a period τ to produce integrated signal. The total integrated charge for each cycle of the accumulator is represented by a digital number, which is coupled over a path 214 to a digital-to-analog converter (DAC or D/A) 15. DAC 15 converts the digital number representing the number of red photons detected during the current interval τ into an analog or analogue signal S(t) proportional to the incident magnetic field component B(t). The analog signal appears at port 216, and may be written as
      S    ⁡          (      t      )        =      k    ⁢                  ∑                  t          =          1                i            ⁢                          ⁢                        μ          h                ⁢                  B          ⁡                      (            t            )                          ⁢        dt            where:
k is a calibration scale factor
is the upper count value of summing index time, t
μ is the Bohr Magneton, a natural physical constant
h is Plancks constant, a natural physical constant
The single-axis Taylor et al. magnetometer described in conjunction with FIGS. 1A, 1B, 1C, 1D, 1E, and 1F is highly sensitive to magnetic fields, at least in part because of the large number of sensing (vacancy) sites per unit volume in the solid diamond lattice. This number of sensing sites much exceeds that of gas-based sensors.